[the5thforce (OP)]

Can the enzymes or other acidic qualities of saliva break down human tissue? Would repetitive contact with saliva slowly thicken tissue or cause reduced sensation?

If the answer is yes, what are the implications for practices such as oral sexual activity?(disregarding the obvious rough surface of the tongue scraping the tissue)

[Bored chemist]

Saliva isn't acidic (it would dissolve your teeth if it was)
Also the best known enzymes in saliva attack starch and the cell wall of bacteria. Since human tissues are not made of starch or from the same stuff that bacterial walls are made there's no sensible risk from saliva.
There's a possible risk from bacteria.
However, human nature hasn't changed much in many centuries and we seem to have survived so far...